The most recent and unexpected example of satellite-detected inflation at a volcano in the United States is the Three Sisters volcanic center in the Oregon Cascade Range, located about 225 km (140 mi) south of Mount St. Helens, Washington.

When testing a new satellite-based technique earlier this year on Cascade volcanoes, Dr. Charles Wicks of the U.S. Geological Survey in Menlo Park, California, discovered a broad area of uplift 15-20 km (9-12 miles) in diameter centered 5 km (3 miles) west of the South Sister volcano.

The volcano is located in the Three Sisters Wilderness of the Deschutes and Willamette National Forests. Satellite images showed that the upplift of about 10 cm (4 inches) occurred sometime between 1996 and 2000.

Word of the Three Sisters uplift spread quickly to Wicks' colleagues at the USGS Cascades Volcano Observatory (CVO) and University of Washington (UW) Geophysics Program, and many questions were raised.

Is the uplift still occurring and at what rate? Can the uplift be verified by some other method? What caused the uplift -- rising magma or increased pressure in the hydrothermal system beneath the area? What needs to be done so that we can provide adequate warning of future volcanic activity if the magma keeps moving toward the surface?

Since the discovery this past spring, USGS and UW scientists have worked with representatives from the Willamette and Deschutes National Forests to develop a skeletal real-time monitoring capability using ground-based instruments in the area.

A continuously operating GPS receiver was installed to identify any sudden change in the uplift rate. A new seismometer was installed in the area so that small earthquakes of magnitude 1 or greater can be detected. Since July, however, only a magnitude-1.9 earthquake has occurred in the area.

In mid-September, scientists also resurveyed benchmarks installed on South Sister in the 1980s. The measured changes are consistent with the uplift determined from the satellite images. In May, a small amount of carbon dioxide volcanic gas was detected in the area, but not enough to warrant concern for public safety. The presence of carbon dioxide is consistent with the idea that magma is responsible for the uplift.

Ever since the eruption of Mount St. Helens in 1980, any hint of possible volcanic activity in the Cascades generates considerable interest among residents of the Pacific Northwest. USGS scientists have met several times with residents and officials in nearby communities and the news media to talk about their most recent findings in the Three Sisters Wilderness.

For now, USGS reports, there is no reason for alarm, and steps are in place for monitoring the area more closely should magma move closer to the surface.

For more information about the discovery and the new satellite technique, see this URL and this one. Both are USGS websites.